The present invention relates to novel polyamide-imides having head-to-tail backbone and more particularly, to novel polyamide-imides having head-to-tail regularity to provide excellent heat and chemical resistance, physical and mechanical properties, processability, and gas permeability and selectivity. The polyamide-imides represented in the formula (1) are obtained by direct polymerization, in the presence of dehydrating catalyst, of alone or as a mixture of two or above of monomers prepared by condensation of an amine compound having a nitro group with a carboxylic acid anhydride. 
wherein Ar1 is selected from the group consisting of 
in which R1, R2, R3 and R4 are the same or different and a hydrogen atom or C1xcx9cC4 alkyl group; and X is selected from the group consisting of xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94C(CH3)2xe2x80x94, xe2x80x94C(CF3)2xe2x80x94, 
Generally, polyamide-imides are prepared by polycondensation of an aromatic tricarboxylic acid anhydride or its derivatives with an aromatic diamine. Therefore, polvamide-imides have alternative arrangement of an amide and imide structures. However, polycondensation of trimellitic acid anhydride or its derivatives as unsymmetric monomers with diamines produce copolymers having a mixed structure of imide-imide backbone or amide-amide (head-to-head or tail-to-tail) and imide-amide (head-to-tail) to have different composition and microstructure because of different reactivities of functional groups in trimellitic acid anhydride or its derivatives. Each structure is shown in the following; 
These microstructures of copolymers are disclosed in Polymer J. 19(8), 667(1983) and J. Polym. Sci., 7, 321(1969).
Commercial polyamide-imides unlike polyimides are injection-moldable amorphous thermoplastic resins having good physical properties. A method of preparing polyamide-imides comprise the polymerization of polyimides with amides and in particular, introduction of amide functional group to polyimides enhance processability. These polyamide-imide resins have randomly mixed structure of head-to-head (or tail-to-tail) and head-to-tail due to no control of the reactivity of an aromatic tricarboxylic acid anhydride (or its derivatives) and a diamine. These random copolymers having disordered arrangement have poor chemical and heat resistance and mechanical properties, thus decrease gas separation properties.
To overcome such shortcomings, intensive studies have been conducted to prepare polyamide-imides having a certain arrangement to provide excellent heat resistance, mechanical properties and gas separation properties.
Polyamide-imides having head-to-head or tail-to-tail backbone are disclosed in German Patent No. 2, 425, 666, Japanese Patent No. 7, 441., 119, J. Polym, Sci., 7, 321(1969) and Macromol. Cliem., 191, 155(1990). However, their heat resistance and other mechanical properties are not improved much.
Another polyamide-imide having head-to-head or tail-to-tail backbone is disclosed in ACS Polymeric Materials Science and Engineering, 77, 289(1997) to improve gas separation property but it is not improved much, either.
It is an object of the present invention to prepare novel polyamide-imides having head-to-tail arrangement structure which provide excellent heat resistance, chemical resistance, physical and mechanical properties and thus enhance gas permeability and selectivity.